1. Field of the Invention
The present invention relates to a lead frame and a semiconductor device using the same and in particular to a lead frame best suited for a semiconductor device having an LOC structure and a semiconductor device using the same.
2. Description of the Related Art
Lead frames for semiconductor devices having an LOC (lead on chip) structure or the like, which can realize high density assembly, are known in the art. One of them is a lead frame used in a system where a high heat-resistant insulating film (usually a polyimide film) coated with a thermoplastic or thermosetting adhesive layer is applied to one side or both sides of the lead frame and a semiconductor chip is mounted on the high heat-resistant insulating film under heating or under pressure.
Application of the high heat-resistant insulating film to the lead frame has hitherto been performed by die stamping application. In this method, a film in a reel form is stamped by means of a die into a desired shape and then applied to the lead frame under heating or under pressure.
The above method, however, poses problems, involved in the use of an adhesive double coated polyimide film, including that (1) the price of the tape is high, (2) the water absorption of the polyimide film is so high that the reliability of a package using such a film is unsatisfactory, and (3) the thickness of the tape is large making it difficult to reduce the thickness of the package.
In order to eliminate the above inconvenience, a method has been proposed which comprises: applying an adhesive alone onto a lead frame in its area where a semiconductor device is to be mounted; and mounting (bonding) the semiconductor device to the lead frame with the aid of this adhesive (for example, Japanese Patent Laid-Open No. 75355/1992).
For the application of the adhesive, a device comprising a combination of an X-Y robot with a dispenser is generally used to apply an adhesive in a varnish form (for example, a solution of an adhesive resin dissolved in a solvent) onto a lead frame in its area where a semiconductor device is to be mounted (for example, a front end of an inner lead). In particular, when an adhesive is applied to the front end of the lead, a spot coating method is used wherein the adhesive is pneumatically delivered through a needle (or nozzle) onto a lead frame in its predetermined area while moving the needle (or nozzle). In this method, what is required is only to coat a required amount of the adhesive onto the lead frame. Therefore, no waste material is produced, and no expensive die is required, advantageously resulting in reduced production cost.
The trend toward mass production of an adhesive coating type LOC lead frame has resulted in a shift of the lead frame from an etching lead frame to a stamping lead frame.
According to the conventional lead frame for LOC, in the case of the etching lead frame, there is no difference in results between coating of an adhesive on one surface and coating of the adhesive on the other surface. On the other hand, in the case of the stamping lead frame suitable for mass production of the lead frame, the face on the punch side and the face on the die side are different from each other in flatness and creation of burrs, leading to a remarkable difference in results of coating between both the faces. In particular, the difficulty, of ensuring insulation, due to a difference in thickness and evenness of adhesive coating and occurrence in burrs of the lead frame is a serious problem. The state of adhesive coating greatly influences the fixation of a semiconductor element to the lead frame. In the stamping lead frame, the face to be coated with an adhesive has not been specified in the art, rendering the provision of an adhesive layer 4 on the burred face unavoidable.